1. Field of the Invention
The present invention relates to a radiographic image conversion screen. More particularly, it relates to a radiographic image conversion screen, i.e. a radiographic intensifying screen (hereinafter referred to simply as "intensifying screen") or a fluorescent screen, which comprises double phosphor layers i.e. a green emitting rare earth phosphor layer and a blue emitting phosphor layer and which has a high speed and exhibits superior image forming characteristics (in this specification, the "radiographic image conversion screen" includes the intensifying screen and the fluorescent screen).
2. Description of the Prior Art
As is well known, a radiographic image conversion screen is used for medical diagnosis and non-destructive inspection of industrial products. The screen emits an ultraviolet ray or a visible ray upon absorption of radiation passed through an object, and thus converts a radiographic image to an ultraviolet image or a visible image.
When the radiographic image conversion screen is used as an intensifying screen for radiography, it is fit on a radiographic film (hereinafter referred to simply as "film") so that a radiation image will be converted to an ultraviolet image or a visible image on the fluorescent surface of the intensifying screen which will then be recorded on the film. On the other hand, when it is used as a fluorescent screen, the radiation image of the object converted on the fluorescent surface of the fluorescent screen to a visible image may be photographed by a photographic camera or may be projected on a television screen by means of a television camera tube, or the visible image thus formed may be observed by naked eyes.
Basically, the radiographic image conversion screen comprises a support made of e.g. paper or a plastic sheet and a fluorescent layer formed on the support. The fluorescent layer is composed of a binder and a phosphor dispersed in the binder and is capable of efficiently emitting light when excited by the radiation of e.g. X-rays, and the surface of the fluorescent layer is usually protected by a transparent protective layer.
For medical diagnosis by means of radiography, a high speed radiographic system (i.e. a combination of a film and an intensifying screen) is desired to minimize the patient's dosage of radioactivity. At the same time, a radiographic system is desired which is capable of providing good image quality (i.e. sharpness, granularity and contrast) suitable for diagnosis by clinical photography. Accordingly, the intensifying screen is desired to have a high speed and to provide superior sharpness, granularity and contrast. Likewise in the case of a fluorescent screen, it is desired to have a high speed and to provide particularly good contrast so that it is thereby possible to minimize the patients' dosage of radioactivity and at the same time to obtain an image having good image quality.
As high speed radiographic image conversion screens, there have been developed radiographic image conversion screens comprising a rare earth oxysulfide phosphor, such as one wherein a terbium-activated rare earth oxysulfide phosphor which is a green emitting phosphor and represented by the formula (Ln, Tb).sub.2 O.sub.2 S where Ln is at least one selected from lanthanum, gadolinium and lutetium, is used (U.S. Pat. No. 3,725,704), and one wherein a terbium-activated yttrium oxysulfide which is a blue emitting phosphor and represented by the formula (Y, Tb).sub.2 O.sub.2 S, is used (U.S. Pat. No. 3,738,856). Among them, intensifying screens using a green emitting rare earth phosphor, particularly, a rare earth oxysulfide phosphor such as a terbium-activated gadolinium oxysulfide phosphor represented by the formula (Gd, Tb).sub.2 O.sub.2 S or a terbium-activated lanthanum oxysulfide phosphor represented by the formula (La, Tb).sub.2 O.sub.2 S, have a speed several times higher than the speed of commonly used conventional intensifying screens using a calcium tungstate phosphor represented by the formula CaWO.sub.4 and they have relatively good granularity as compared to other high speed intensifying screens. Therefore, they are utilized in high speed radiographic systems in combination with an orthochromatic-type (hereinafter referred to simply as "ortho-type") film having a wide spectral sensitivity ranging from a blue region to a green region. Meanwhile, in the recent high speed radiographic systems based on a combination of a green emitting rare earth intensifying screen and an ortho-type film, there is a tendency to use a low speed ortho-type film utilizing fine silver halide grains in order to minimize the amount of silver used for the film and to improve the image quality, particularly the granularity, at a high speed level. It is therefore strongly desired to further improve the speed of the intensifying screen with a view to reduction of the patients' dosage of radioactivity and at the same time to improve the sharpness of the intensifying screen, which tends to be reduced with an increase of the speed.
Among the green emitting phosphors, a gadolinium oxysulfide phosphor is particularly preferably used for a high speed intensifying screen. However, it has a K absorption edge at 50.2 KeV, and accordingly, the intensifying screen using it has drawbacks that the contrast thereby obtainable within the X-ray tube voltage range commonly used for medical diagnosis (i.e. from 60 to 140 KV.sub.p) is inferior due to the X-ray absorbing characteristics of such a phosphor. Moreover change of the speed of the intensifying screen changes as a function of changes in the tube voltage, which changes can be substantial thus leading to difficulties in determining the condition of radiography.